Novel Ceramic Tile Designs for Armor

ABSTRACT

A ceramic-containing armor tile exhibiting anisotropy exhibits different anti-ballistic performance compared to a uniform, isotropic tile. The ballistic performance has been quantified, and the results suggest that design can be optimized for even greater performance.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support under Contract No. W911QY-08-C-0093 awarded by the Department of the Navy. The Government has certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

None.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ceramic-containing armor for defeating ballistic projectiles. More particularly, the present invention relates to the design, that is, the specific geometric features contained in the armor, and more specifically, it relates to the effect of non-uniform features.

2. Discussion of Related Art

Ceramic tiles are valuable components in armor systems for the defeat of armor piercing projectiles. Because of their high hardness, ceramics effectively break or blunt the projectile, thus allowing remaining portions of the armor system (e.g., metallic and/or polymeric backing materials) to catch the debris.

Metals (steel, aluminum, titanium, etc.) are also commonly used for armor applications, as they offer significant cost advantages over ceramics. However, due to their relatively low hardness, their effectiveness on a weight basis is poor relative to ceramics.

To increase the effectiveness of metals for armor applications, significant work has been done with novel geometries. Examples include perforations, angled holes, blind holes, etc.

U.S. Pat. No. 3,736,838 to Butterweck et al. discloses a protective or chain shield or apron for armoring a vehicle. The protective shield includes a perforated steel plate surrounded by a rubber layer or rubber frame. The perforations consist of four holes arranged to define a quadrangle, with a fifth hole at the intersection of the diagonals of the quadrangle.

U.S. Pat. No. 4,835,033 to Auyer at al. discloses an armor plate including a hardened steel plate having triangular holes arranged in a repeating pattern. The triangular holes generally have the same size and shape, which is preferably that of an equilateral triangle. “Webs” of the hardened steel material between the holes are generally straight.

U.S. Pat. No. 5,007,326 to Gooch, Jr. et al. discloses a defensive appliqué armor for protecting a substrate from ballistic projectiles. The armor includes one or more cast metal plates of predetermined thickness which contain slotted holes of various sizes (but smaller than the caliber of a projectile) and designs. The slotted holes are at an oblique angle relative to the surfaces of the plate. The cast armor may contain an optional, thin cast metal layer within or external to the cast armor plate.

U.S. Pat. No. 5,014,593 to Auyer et al. discloses perforated plate armor featuring inner and outer perforated steel plates, each plate featuring an associated pattern of holes. The perforated steel plates are heat treated to have hardened surfaces and a more ductile core. The plates are spaced with respect to each other at outer and inner locations with respect to the object to be protected. The patterns of holes in each layer are offset with respect to one another to prevent straight line penetration of any particle or projectile through both plates. An inner backing plate is provided to stop any projectiles that might penetrate both plates. Fillers and connectors space the inner and outer steel plates with respect to one another.

U.S. Pat. No. 5,221,807 to Vives discloses an armor system for providing ballistic protection, the system featuring an armor plate with an auxiliary plate placed at a predetermined location in front of the armor plate. The armor plate may be a ceramic matrix composite (CMC), or it may be a two-part assembly consisting of a sintered ceramic for the front piece and CMC, Kevlar or steel for the rear piece. The auxiliary plate may be made of a sintered ceramic or of a fiber and CMC. The auxiliary plate features a plurality of circular, blind holes, smaller in diameter than the caliber of the projectile, the holes opening to the front (impact) surface, but not to the rear surface. The holes are cylindrical or possibly slightly conical. The holes are distributed in rows and columns to perform a regular mesh over the front face of the auxiliary plate. In an alternate embodiment, instead of the front face of the auxiliary plate being planar, it may be given an irregular shape made up of zones of relief inclined at various angles and sloping in various directions. For example, the front face made by made up of a plurality of pyramids, or rows of parallel fluting.

U.S. Patent Application Publication No. US 2006/0060077 A1 to Lucata et al. discloses a ceramic armor system featuring an integral or mosaic ceramic plate, a front spall layer bonded to the front surface of the ceramic plate, a shock-absorbing layer bonded to the rear surface of the ceramic plate, and a backing that is bonded to the exposed face of the shock-absorbing layer. The ceramic plate may have a flat front surface or a deflecting front surface. The deflecting front surface preferably is provided with a pattern of multiple nodes, whose configuration may be spherical, cylindrical or conical. The nodes may be mono-sized or may have a bi-modal size distribution. One or more nodes may feature a longitudinal channel, thereby lowering the areal density of the armor.

SUMMARY OF THE INVENTION

In accordance with the instant invention, the effects of specific geometric features within the tile on ballistic behavior have been quantified. The findings are new and unique relative to the prior art. Specifically, the key findings are:

-   -   Solid strike face is preferred to provide desired interaction         with projectile (i.e., through perforations not desired).     -   Lightweighting features (e.g., blind holes) desired on back face         to allow tile thickness (and thereby stiffness) to be increased         without adding weight.     -   Back face features with geometry that prevents stress waves from         reflecting directly back to the front face. An example is blind         conical perforations on tile back face that will reflect the         stress wave at an angle.     -   Features on back face of tile (i.e., tensile bending stress side         of tile upon impact) that will act as crack arresters. An         example is any circular depression that will dramatically         increase the crack tip radius when a propagating crack enters         the geometric region.     -   Anisotropic tile geometry to induce projectile to turn or tip         upon impact. Geometry can include front face “irregularity”         and/or back-face blind perforations.

Quantifying the effects of the various geometries is significant because it gives rise to the ability to optimize performance.

In addition to the above quantitative or numerical data, the present work has demonstrated the following technical highlights of a novel tile design relative to traditional solid tiles of uniform cross-section:

-   -   Limited collateral damage due to the presence of “crack         arresting” geometries within tile.     -   Ability to tip or turn projectile due to anisotropic geometry.     -   Increase in thickness (and thereby stiffness) of tile due to         presence of lightweighting geometries within tile.

As a result of these technical advantages, increased ballistic performance is seen.

DEFINITIONS

“Anisotropy”, as used herein, means that an armor tile presents an uneven surface to an impacting ballistic projectile, or presents a volume of material encountered by the projectile that is non-uniform in its amount, arrangement or properties.

“Preform” as used herein, means a shaped, self-supporting ceramic-containing body having interconnected porosity.

“Tile”, as used herein, means a ceramic-containing body for an armor application, but not combined with other constituents often required to make an effective armor system, such as a spall layer, and intermediate layer, a backing layer, etc.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the various geometries studied in the present work. In particular,

FIG. 1 a is an isometric view of an armor tile containing an ordered arrangement of through holes;

FIG. 1 b is an isometric view looking toward the rear surface of an armor tile containing an ordered arrangement of blind holes;

FIG. 1 c is an isometric view looking toward the rear surface of an armor tile containing an ordered arrangement of blind conical holes; and

FIG. 1 d is an isometric view looking toward the rear surface of an armor tile containing an ordered arrangement of non-conical blind holes and whose front surface (“strike face”) features bumps or knobs;

DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS

The ballistic performance of an armor is a function of the nature and amount of materials making up the system. In addition, the performance is also a function of the way in which the materials are arranged or presented; that is, their geometry or design. In a ceramic-containing tile, for a given amount of tile material, ballistic performance will vary depending on how that material is presented to the impacting projectile, e.g., whether there are surface features on one or both sides of the tile.

Without wishing to be bound to a particular theory or explanation, intentionally designing anisotropy or non-uniformities into an armor tile may enhance anti-ballistic performance. For example, the one or more anisotropies may deflect, tip or turn the impacting projectile. Deflection could result in a longer dwell; that is, the projectile may encounter a greater volume of material, and may have insufficient energy to penetrate the greater amount of material. Tipping or turning the projectile may be useful because it may increase the impact area which would decrease the impact pressure. The increased impact area may result in greater total crack surface. As energy is required to create a crack, the greater crack surface area represents a greater amount of energy absorbed by the armor from the projectile.

It was seen previously that through-holes, that is, holes that extend completely through a metal armor tile (there called a “plate”) enhanced ballistic performance, at least on a weight basis. In the present work with ceramic-containing materials, however, armor tiles containing such through holes performed much worse ballistically than a solid uniform cross-section tile containing no surface features, as will be seen in more detail in the Example below. Thus, one cannot extrapolate performance in one armor system such as one based on monolithic metal armors, to performance in a different system, e.g., ceramic-containing armors.

Thus, the present work focused on other areas, such as whether partial or “blind” holes, the shape of such holes, and whether anisotropies such as surface features on the impact surface of the tile can enhance performance. The results of this work, to be discussed in more detail below, indicate that these variables indeed affect ballistic performance, that such performance can be quantified, and that further optimization work would be useful.

Blind holes are typically thought of as being open on one end, but they can also be completely enclosed. For example, a tile containing a plurality of blind holes open on a common side of the tile can be covered on that side with a plate that is then attached or otherwise bonded to the tile.

The cross-section of the blind holes may be that of a circle, parabola, ellipse, hyperbola, polygon or a portion thereof. Thus, the holes may have straight or planar sides. However, the intersections of planes or lines should not come to a sharp point but rather should feature a radius. Sharp edges or points are stress risers in brittle materials such as most ceramics.

The “tile” is the ceramic-containing body whose job it is to fracture and/or comminute (that is, pulverize or grind) the impacting ballistic projectile. The tile may be solid or monolithic ceramic or a composite such as a metal matrix composite (MMC) or ceramic-matrix composite (CMC), or a composite containing both interconnected metal and ceramic.

Popular armor ceramic materials include silicon carbide, boron carbide, and aluminum oxide. Other ceramics effective in armor applications include titanium diboride, boron nitride, diamond, silicon nitride and SIALON, a silicon-aluminum oxynitride.

Processing techniques for making dense ceramic-containing bodies include sintering, hot pressing and infiltration of gases (e.g., chemical vapor infiltration) or liquids (e.g., molten metal), in addition to the many techniques known in the art for making preforms. Aluminum and its alloys is a popular metal that is infiltrated in molten form to make aluminum matrix MMCs.

Another common liquid infiltrant is molten silicon and its alloys. When infiltrated, typically under wetting conditions, with little or no chemical reaction, the process is termed “siliconizing”, and is exemplified by U.S. Pat. No. 3,951,587 to Alliegro. More common is to chemically react at least a portion of the silicon metal (and for purposes of this document silicon is considered a metal) with some reactable or “free” carbon contained in the preform to produce some silicon carbide, typically interconnected, in the resulting dense body. Such process goes by various names including reaction bonding, melt infiltrating, reactive sintering, and reaction forming. The preform usually contains one or more hard ceramic “filler” materials that contribute high stiffness and hardness but are relatively inert with respect to the molten silicon or alloy. When the preform contains predominantly silicon carbide, the product is called “reaction bonded silicon carbide”, and when the preform contains a significant amount of boron carbide, the densified product is called “reaction bonded boron carbide”. See, for instance the “Examples” sections of U.S. Pat. Nos. 7,104,177 and 6,862,970, respectively, which sections are expressly incorporated herein by reference.

The following example illustrates with still more specificity several embodiments of the present invention. This example is meant to be illustrative in nature and should not be construed as limiting the scope of the invention.

Example

This Example demonstrates the effect on ballistic performance of tile geometry or design. In other words, this example compares the ballistic performance of five ceramic-containing armor systems where the only difference is that one system featured a ceramic-containing tile of solid material of uniform cross-section (the control), and the other tiles featured a non-uniform strike face and/or non-uniformity in parts of the tile other than the strike face.

In addition to a baseline or “control”, four different tile designs were tested. The baseline tile was a solid, relatively smooth tile not containing any holes, depressions or projections, similar to what is described in Example 1 of U.S. Pat. No. 6,862,970. As illustrated in FIGS. 1 a-1 d, the other four designs consisted of

-   -   Tile with through perforations     -   Tile with solid front and blind holes on back     -   Tile with solid front and blind cones on back     -   Tile with knobby front and blind holes on back

From each design, four to eight tiles were made from reaction bonded B₄C ceramic. The compositions and procedures described in Example 1 of U.S. Pat. No. 6,862,970 are satisfactory for producing such tiles. The tiles had a multi-curved geometry and measured nominally nine by twelve inches (about 23 by 30 cm). The thickness of each tile was controlled such that all tiles had nominally constant weight despite the geometry change.

Each tile was backed with constant weight of armor-grade ballistic polymer (Honeywell Spectra Shield).

For each design, ballistic testing was conducted per MIL STD 662 to determine V₅₀ vs. a 7.62 mm AP round (V₅₀ is a statistical measure of the performance of an armor system, and represents the velocity of the projectile at which 50% of the projectiles will be stopped). For the present document, the V₅₀ results are presented as a velocity relative to the V₅₀ of the solid baseline tiles.

The results, shown in Table 1, demonstrate that tile design can have a dramatic effect on performance, both in the positive and negative direction.

TABLE 1 Ballistic Results V₅₀ Result vs. 7.62 mm Tile Design AP Projectile Standard Solid Tile Baseline Through Perforations Baseline minus 1250 ft/s Solid Front, Blind Holes on Back Baseline minus 279 ft/s Solid Front, Blind Cones on Back Baseline plus 70 ft/s Knobby Front, Blind Holes on Back Baseline plus 193 ft/s Knobby Front, Blind Cones on Back Future work

A summary of key findings is:

-   -   Holes on front face not desired.     -   Cones on back face are superior to blind holes.     -   Irregular (non-flat) geometry on front face helpful.     -   Opportunity for improvement exists—e.g., combination of         irregular front face and blind cones on back face.

In addition to the above quantitative or numerical data, the present work has demonstrated the following key technical advantages of a novel tile design relative to traditional solid tiles of uniform cross-section:

-   -   Limited collateral damage due to the presence of “crack         arresting” geometries within tile.     -   Ability to tip or turn projectile due to anisotropic geometry.     -   Increase in thickness (and thereby stiffness) of tile due to         presence of lightweighting geometries within tile.

As a result of the quantitative and non-quantitative (or qualitative) results, the investigators drew the following conclusions:

-   -   Solid strike face is preferred to provide desired interaction         with projectile (i.e., through perforations not desired).     -   Lightweighting features (e.g., blind holes) desired on back face         to allow tile thickness (and thereby stiffness) to be increased         without adding weight.     -   Back face features with geometry that prevents stress waves from         reflecting directly back to the front face. An example is blind         conical perforations on tile back face that will reflect the         stress wave at an angle.     -   Features on back face of tile (i.e., tensile bending stress side         of tile upon impact) that will act as crack arresters. An         example is any circular depression that will dramatically         increase the crack tip radius when a propagating crack enters         the geometric region.     -   Anisotropic tile geometry to induce projectile to turn or tip         upon impact. Geometry can include front face “irregularity”         and/or back-face blind perforations.

INDUSTRIAL APPLICABILITY

The ceramic-containing armors of the instant invention, possessing the desirable properties of low specific gravity and high hardness, should be particularly useful against ballistic threats such as small arms fire, e.g., as body armor, and as aircraft armor. The instant armors might also find application in marine vessels and ground-based vehicles, e.g., for armor protection against heavier threats.

The techniques and technologies disclosed in the present work may be applicable to other industries, for example, those benefiting from lightweighting efforts of materials that are inherently hard and stiff Thus, other applications in such industries as the precision equipment, robotics, optics, tooling, aerospace, electronic packaging and thermal management, and semiconductor fabrication industries, among others, will occur to those skilled in those arts.

An artisan of ordinary skill will appreciate that various modifications may be made to the invention herein described without departing from the scope or spirit of the invention as defined in the appended claims. 

1. A tile for an armor system for use against ballistic projectiles, the tile comprising: (a) a ceramic-containing material; (b) a front face and a rear face; and (c) a plurality of holes that are not open to the front face.
 2. The tile of claim 1, wherein at least some of said holes are open to said rear face.
 3. The tile of claim 1, wherein said holes are spherical.
 4. The tile of claim 1, wherein said holes are cylindrical.
 5. The tile of claim 1, wherein said holes are conical.
 6. The tile of claim 5, wherein said conical holes define a cone in cross-section, and further wherein a base of said cone is located at said rear face.
 7. The tile of claim 1, wherein said holes taper toward the front face.
 8. The tile of claim 1, wherein said front face is flat.
 9. The tile of claim 1, wherein said front face is not flat.
 10. The tile of claim 9, further comprising projections that extend outward from said front face.
 11. The tile of claim 10, wherein said projections comprise mounds, bumps or knobs.
 12. A tile for an armor system for use against ballistic projectiles, the tile comprising: (a) a ceramic-containing material; (b) a front face featuring anisotropy; and (c) a plurality of blind holes.
 13. A tile for an armor system for use against ballistic projectiles, the tile comprising: (a) a ceramic-containing material; (b) a front face and a rear face, the rear face being arranged to reflect shock waves away from direct impingement on the front face during ballistic impact of a projectile onto said front face.
 14. A tile for an armor system for use against ballistic projectiles, the tile comprising: (a) a ceramic-containing material; and (b) a front face and a rear face, the front face featuring anisotropy. 